Role of Aluminum Nitride

2.7.3.1. Role of Aluminum Nitride

The change in bulk density of the sintered body as a function of aluminium nitride concentration was measured at different sintering temperatures. The results are shown in Figure 2.15, where it seen that with an increasing AlN content, the density increases up to a maximum of 3.16 gm/cc, or 98.4% of TD, i.e. a maximum sintered density is obtained at a concentration of 2 wt% AlN at each temperature from 2000° – 2100°C. Above 2 wt% AlN concentration, the density decreases from its peak value. As shown later, on a molar basis, this almost corresponds to the same amount of boron, which was found to

be an ‘optimum concentration’ of dopant. In order to achieve high sintered density, the simultaneous addition of carbon is necessary like in the case of boron addition. These results were obtained at a constant amount of carbon addition, i.e. 1 wt%.

SILICON CARBIDE

Y( 80

IT DENS

2104°C

2050°C 2000°C

CARBON = 1 wt%

ain (wt%)

Figure 2.15: Bulk density of sintered α-SiC against AlN content. It is a significant result that the sintered density changes from 62% of TD to 99% of TD at 2

wt% AlN at a temperature of 2100°C for a time of 15 mins under vacuum (3 mbar). These results suggest that the solid solubility of Al in SiC may be 1 wt% from 2000° - 2100°C. As a matter of fact, the body having 99% TD exhibits a dense microstructure with relatively fine grains, the average size of which is around 3.7 µm. Two types of porosity could be identified in the microstructure :

(a) Between the grain boundaries, and (b) Within the bulk crystal The latter were developed during the grain growth of the original nano-sized SiC grains. The

needlelike growth of the grains of SiC points out to the fact that the rate of crystallization from the original (rather spherical) particles, produced by grinding and leaching etc., has been very fast favouring the growth in a particular direction, according to the energy consideration. Due to the growth from 37 nm to 5700 nm (Figure 2.16), it is suggested that a group of grains after sintering should have the grain boundaries with a relatively low energy.

NANO MATERIALS

Figure 2.16: TEM photo of sintered α-SiC doped with aluminium nitride. Since the sintering takes place through a ‘diffusion’, the following mechanism for the enhance-

ment of diffusion is proposed as [2] :

SiC ⎯⎯⎯⎯⎯⎯→ Si + C AlN

SiC ⎯⎯⎯⎯⎯⎯→ Al Si ′′′ +N C ′′′′ SiC Si 3 N 4 ⎯⎯⎯⎯⎯⎯→ 3Si ′′′ + 4N′′′ + V Si Al 4 C 3 ⎯⎯⎯⎯⎯⎯→ 4Al′′′ + 3C′′′ + V C

AlN and SiC form solid solution for a limited range. When aluminium enters into Si site, Al 4 C is likely to be formed leading to the creation of carbon vacancy, whereas when nitrogen enters into the carbon site, Si 3 N 4 is likely to be formed, which leads to the formation of a silicon vacancy. Thus, the vacancies are created, which lead to the increase of the diffusion coefficient of both silicon and carbon. In argon atmosphere, the carbon diffusion coefficient is two orders of magnitude larger than that of silicon. However, in carbon rich atmosphere, the carbon diffusion coefficient is less than the diffusion coefficient of silicon, and also the enhancement of the diffusion coefficient of silicon occurs, as reported by Rijswijk and Shanefield [42]. The details of this mechanism working for the better densification of nano-particles of SiC is given in some more details in the section - 2.7.3.3.